The present invention relates to the preparation of concentrated hexafluorophosphoric acid (HPF6) complex solutions containing water.
Known processes for the production of hexafluorophosphoric acid include:
1. H3PO4+6 HFxe2x86x92HPF6+4 H2O
2. P2O5+12 HFxe2x86x922HPF6+5 H2O
3. 2H3PO4+6 CaF2xe2x86x92PF5+HPF6.2H2O+HF+6Ca SO4 
The preparation of hexafluorophosphoric acid is usually accomplished by the addition of anhydrous HF to phosphoric acid or phosphorus pentoxide. The preparation using pure phosphorus pentoxide is difficult since it is a dry air sensitive powder and therefore some liquid phase is usually added to facilitate blending and reaction with the HF. This results in a H2O/HPF6 ratio of about 3-4, never achieving the theoretical 2.5 ratio of water to HPF6 when using P2O5.
U.S. Pat. No. 3,634,034 to Nickerson et al, which relates to process 3, produces along with phosphorous pentafluoride, HPF6 and calcium sulfate. Separation problems and disposal of large amounts of calcium sulfate must be addressed with the process.
HPF6 is not stable at ambient conditions without a stabilizing coordination complex like water. In fact, very little or no PF6 is absorbed when bubbled into anhydrous HF at 0xc2x0 C. showing that no complex is formed without water being present.
The presence of excess water is undesirable in solutions of hexafluorophosphoric acid because it promotes hydrolysis of the PF6 anion to partially oxygenated species. In addition, it decreases the overall effectiveness and acidity of the acid and dilutes the concentration of the acid. It also adds extra shipping weight to cost. The pure hexafluorophosphoric acid has not been reported under ambient conditions (atmospheric pressure and room temperature).
Therefore, there exists a need to provide high purity HPF6 in high concentrations that is stable at ambient conditions and can be prepared in a simple and economical procedure.
According to the present invention, there is provided a stable hexafluorophosphoric acid complexed with water having about 1.0 to 3.3 molecules of water to one hexafluorophosphoric acid molecule at 20xc2x0 C. and the process for its preparation.
According to one embodiment of the invention, there is prepared a hexafluorophosphoric acid composition compound having at least one molecule of water per hexafluorophosphoric acid molecule at temperatures below 20xc2x0 C., preferably 1.6 to 1.7 molecules of water to one hexafluorophosphoric acid molecule.
According to another embodiment of the invention, there is further prepared a 1.7 mole ratio of H2O/HPF6 complex which is about 83% HPF6 concentration from polyphosphoric acid.
According to a further embodiment of the invention hexafluorophosphoric acid monohydrate is prepared.
Therefore, an object of the present invention to provide an improved process for the preparation of a stable high purity hexafluorophosphoric acid complex in high yields.
Another related object of the present invention is to provide a process which is highly efficient and economical.
A further object of the present invention is to provide a process which produces a minimum amount of by-products.
It has found that to stabilize HPF6, adding only enough water to maintain its vapor pressure at about that of one atmosphere is essential. Preferably, the amount of water necessary is at least one molecule of water per hexafluorophosphoric acid molecule below 10xc2x0 C. and more preferably 1.6-1.7 molecules of water to one hexafluorophosphoric acid molecule at about 20xc2x0 C. This solution can be kept in a Teflon bottle, polyethylene bottle or in a steel low-pressure cylinder.
Extra HF can also be added, but it does not contribute significantly to the stabilization of the PF6 anion at these high concentrations. Since it is desirable to have as little water as possible and maximize the highest concentration of the hexafluorophosphoric acid, the preferred ratio is about 1.6-1.7. The highest HPF6 concentrations available from other conventional routes can produce water/hexafluorophosphoric acid ratios only greater than 3.3 on a practical basis.
According to the present invention, one is able to produce these concentrated solutions of hexafluorophosphoric acid (ratios from 1 to 3.3) by adding PF5 gas with cooling to various concentrations of HF in water (from 25% to 52% by weight HF) to produce the desired low ratios or amounts of water in the product. The phosphorous pentafluoride can contain phosphorous oxyfluoride (POF3).
According to a further embodiment of the invention, one can produce these same high concentrations of hexafluorophosphoric acid (low water, 11-28% by weight) by adding PF5 gas to already prepared solutions of hexafluorophosphoric acid which have water ratios higher than 3 and which also contain the calculated amount of excess HF to form the desired concentration of hexafluorophosphoric acid having low water (ratio of 1 to 3).
According to the present invention, a stable colorless crystalline 1:1 HPF6/H2O complex forms below 10xc2x0 C. when PF5 is bubbled into essentially 50% HF/water below 10xc2x0 C., namely 0-5xc2x0 C. This complex appears to melt with decomposition in the 10-12xc2x0 C. range. This crystalline complex also forms a slurry in solutions with the H2O/HF ratio from 1 to 2 below 10xc2x0 C. These solid complexes decompose giving off PF5 when the solutions are warmed to 20xc2x0 C. (room temperature). The equilibrium concentration in a closed bottle (fluorocarbon or polyolefin) at approximately one atmosphere is a mole ratio of H2O/HPF6 of 1.6-1.7 (about 83xc2x12% by weight HPF6 and 17xc2x12% water).
According to the present invention, the process is as follows:
1) PF5+HF(1 to 3.3 moles water for each HF)xe2x86x92
The ratio of H2O/HPF6 is 11-29. HPF6+1-3.3 H2O
2)PF5+HPF6/3.33 H2O+HFxe2x86x92HPF6+1-3.3 H2O
The ratio of H2O/HPF6 is 11-29.